Function process

test/anyflow/test_dependency.cpp:22–25

struct MutableProcessor : public GraphProcessor {
  virtual int process() noexcept override {
    *x.emit() = *a;
    return 0;
  }

  ANYFLOW_INTERFACE(ANYFLOW_DEPEND_MUTABLE_DATA(::std::string, a)
                        ANYFLOW_EMIT_DATA(::std::string, x))